I am a female about 5'1, 20 years old, and 90-98 pounds (depends on how my body feels i guess). how much should i weigh?

It depends: There is a widely variable range of weights for healthy patients. You seem to be on the smaller side which may be your normal baseline! However, if you are experiencing symptoms such as fatigue and joint pain see your doctor for a proper evaluation.

Two possibilities...: A BMI for height 5’1”, 90lbs is 17, for 98lbs is 18.5. On the basis of height and weight alone, a BMI of 18.5 is at the lower end of what is considered healthy. Interpretation of the association with BMI with poor health, however, requires a consideration of whether the relatively low BMI is due to a disease process associated with under nutrition or other chronic illness as opposed to physically